1. Field of the Invention
This invention relates to flow control apparatus and in particular, to a flow control apparatus having a noise and vibration suppression arrangement associated therewith.
2. Description of the Prior Art
In general, a steam turbine power plant, whether fossil or nuclear, comprises a series-connected arrangement of a steam generator element, turbine element, and a condenser element. It is, of course, well known that the turbine element converts the energy contained within a high pressure and high temperature motive fluid, commonly steam, into rotational mechanical energy. The rotational mechanical energy is transmitted by a common shaft to an associated electrical generator element which provides electrical energy to an electrical load. Suitable flow control apparatus, including at least one control valve, is connected within the conduit arrangement between the steam generator and the turbine elements.
The control valve, in addition to providing steam interdiction capability in the event of system malfunction, has as its function the regulation of the steam flow from the steam generator element into the turbine element. The steam conducted into the turbine element enters through an admission arc which is located within the turbine immediately preceding the first array of stationary blade elements mounted within the turbine casing. It is the practice in the art to provide, for certain turbine constructions, a plurality of admission arcs and, for this reason, such turbines are said to have a plurality of partial admission arcs therein. If a turbine is provided with such a plurality of partial admission arcs, there is a control valve disposed in the steam line between each admission arc and the steam generator element.
When low load conditions are imposed upon the generating system, usually during off-peak periods, it is common practice to reduce the mass flow of motive fluid to the turbine. To accomplish this reduction in mass flow to the turbine, the position of a valve plug within the control valve is modulated relative to its valve seat. Such modulation reduces the pressure of the motive fluid entering the turbine, and since, in single admission arc turbines, the admission area of the turbine remains constant, the mass flow rate entering the turbine is reduced. Of course, if a turbine having a plurality of partial admission arcs is utilized, reduction of mass flow rate is accomplished by sequentially reducing the area available to the flow; that is, utilizing only a predetermined number of the admission arcs available. Further, the pressure of the motive fluid entering each partial admission arc is regulated by modulating the position of the control valve plug associated with and governing the flow into each admission arc.
It has been observed in some valve constructions that, for a given valve plug position, the pressure ratio between the static pressure at the valve inlet and the static pressure at the inlet of the turbine is lower than a predetermined value, the fluid streams entering the outlet conduit beneath the valve plug flow along the conduit boundaries and do not separate therefrom. It has also been observed for such a flow pattern that a gradual increase in the static pressure occurs within the outlet conduit for approximately five pipe diameters downstream of the valve plug. The increase in the static pressure downstream from the control valve creates a pressure in the outlet conduit downstream of the valve plug that is greater than the static pressure immediately underneath the valve plug. Thus, for low valve-inlet-to-turbine pressure, ratios the static pressure downstream of the valve plug is greater than the static pressure immediately underneath the valve plug. A back flow of fluid in the outlet piping is thereby produced. This back flow has the advantageous effect of dissipating the velocity of the inlet flow stream. Although the interaction of the recirculated back flow and the high velocity inlet stream causes some shear noise, for lower inlet pressure ratios these effects are not deleterious.
It has been found, however, that during low load conditions, the ratio of valve inlet pressure to turbine inlet pressure for a given valve plug position exceeds the predetermined value. During such a condition, it has been observed that excessive noise and vibration levels occur within the control valve. Such excessive noise and vibration levels are thought to be generated by the impingement of the high velocity motive fluid streams beneath the valve plug adjacent to the valve outlet.
The fluid streams enter the channel defined between the valve plug and valve seat at such a high velocity that separation from the boundary walls of the outlet conduit occurs, and fluid streams entering from opposite sides of the valve plug impinge beneath the plug. The impingement of the fluid streams generates closed vortex immediately beneath the valve plug. Fluid trapped within the vortex exerts time varying forces against the underside of the plug, which cause excessive vibration within the valve. In addition, the collision of the influent streams generates an excessive noise level within the valve.
As a result of the flow on the axis of the outlet conduit, the natural back flow of recirculating fluid (which occurs for lower pressure ratios) is blocked and forced to the outer walls of the outlet conduit. Thus, for the higher valve-inlet-to-turbine pressure ratios which occur during low load conditions, viscous interaction between the influent flow streams and the recirculating back flow is reduced greatly.
It is apparent that the noise and vibration induced by the impingement of the inlet flow stream must be controlled in order to prevent possible damage to the valve itself and to the associated conduits. In addition, operating noise levels must meet recently promulgated safety standards. It is therefore imperative, both from an operational and an environmental standpoint, that the noise and vibration levels presently being generated be abated.
The prior art has attempted to suppress the noise and vibration within the control valve by a variety of methods. One method disposed an annular muffler circumferentially about the valve plug. The muffler has extending therethrough a plurality of small bores. The muffler is disposed within a predetermined close clearance of the valve plug such that any influent flow stream entering the channel defined between the valve plug and the valve seat is required to pass through the plurality of openings. It is anticipated that fluid friction between the walls of the bores in the muffler and the influent fluid stream can sufficiently dissipate the velocity of the motive fluid to prevent the influent streams from colliding beneath the valve plug. However, it is empirically found that it is impossible to provide a sufficient number of muffler bores having a sufficiently small diameter in order to effectively dissipate the velocity head of the influent motive fluid stream.
As an alternative, the turbine power plant may be operated in a mode such that the ratio of input pressure of the valve relative to the input pressure of the turbine is kept below the predetermined value so that the above-described impingement of the inlet flow streams does not occur. However, maintaining the inlet pressure ratio below this predetermined threshold has a deleterious effect upon the operating efficiency of the entire system.
It can therefore be appreciated that a flow control apparatus having a noise and vibration suppression arrangement associated therewith to inhibit the noise and vibrational levels currently being generated within the control valves presently utilized must be provided.